Implantable cardioverter-defibrillator (ICD) systems comprise a subcutaneous generator connected to the heart by a transvenous, multilumen defibrillation lead. The lead's insulated cables carry physiological electrical signals from the heart to the generator and, when necessary, transmit therapeutic shocks from the generator to the heart to correct life-threatening arrhythmias. Leads comprise the weakest link in ICD systems, as evidenced by recalls affecting hundreds of thousands of patients. Lead-failure diagnostics in present ICDs and those under development suffer from inherent and irremediable limitations, especially for detecting insulations breaches, the most common cause of lead failure. Thus insulation breaches commonly present with serious complications. Our objective is to develop a novel instrument that detects subclinical lead failures accurately and is suitable for evaluating in-situ leads. The principle behind this instrument is that subclinical physical changes in insulation alter the capacitance between lead conductors and external conductive blood and tissue, resulting in changes in high-frequency (HF) measurements of impedance. In our phase 1 study, we confirmed that insulation breaches alter HF impedance using both novel electromagnetic (EM) modeling and in-vitro testing. In the present study, we propose (1) to expand both EM modeling and in-vitro testing, (2) to construct an instrument that applies high-frequency lead integrity (HFLI) testing to implanted leads, and (3) to test this instrument in animals. Such an instrument would fulfill a significant unmet clinical need. At ICD generator change, accurate identification of leads that will fail within the anticipated 10-year service lifeof new generators would permit prophylactic lead replacement, reducing both patient morbidity and costs from lead failures and risks and costs of multiple operations. Ultimately, HFLI technology could be incorporated into the ICD generator's circuitry. For new lead designs, HFLI testing could play a critical role either to confirm expected performance or identify subclinical insulation problems, permitting a problematic lead to be withdrawn at an earlier stage.